Defect inspection of semiconductor wafer and mask in IC manufacturing is an accepted significant production process for yield enhancement. The information obtained from a wafer defect inspection tool can be used to flag defective dies for repair, or improve wafer processing parameters. There are two main kinds of inspection tools used for defect inspection in semiconductor fabrication plants, i.e., optical inspection tool and electron beam inspection tool. The spatial resolution of an optical inspection tool is fundamentally limited by its larger wave diffraction spot because an optical wavelength is much longer than that of an electron beam. As critical dimensions of patterns on a wafer and a mask are required to become smaller and smaller, it is becoming incompetent. However, the throughput or called as inspection speed of an electron beam inspection tool is fundamentally limited by electron interaction or called as Coulomb Effect. As inspecting full wafer or full mask is becoming necessary, increasing its throughput is becoming a key issue. As a promising solution, using a plurality of electron beams to inspect wafer in parallel was provided many years ago.
The first patent using a multi-axis magnetic lens to separately focus several parallel electron beams was granted to Maekawa et al. in 1973 for throughput improvement of an IC pattern exposure system. The apparatus includes one common exciting coil 44, one york 43, and two magnetic conductor plates 41 and 42 with a plurality of through holes 1, 2 and 3 for the corresponding charged particle beam passing, which was proposed in U.S. Pat. No. 3,715,580 and is illustrated in FIG. 1A. Between a pair of aligned holes in the upper and lower magnetic conductor plates 41 and 42, a sub-lens such as 10 is formed, so as sub-lenses 20 and 30. The two magnetic conductor plates 41 and 42 are the pole pieces of these sub-lenses 10, 20 and 30. In this multi-axis magnetic lens, the magnetic fields of the sub-lenses are fundamentally different from each other in distribution pattern and strength as shown in FIG. 1B.
Comparing with a conventional single-axis magnetic lens, one main problem of this multi-axis magnetic lens is the magnetic field distribution of each sub-lens degenerates from axial symmetry to rotation symmetry and/or n-fold symmetry (FIG. 1B). As a result, besides an axisymmetric field or called as round lens field which focuses a particle beam, a lot of undesired non-axisymmetric transverse field components or called as high order harmonics in terms of Fourier analysis of magnetic field, such as dipole field and quadrupole field appear. The dipole field deflects the charged particle beams, makes the beam land on the imaging plane with an additional transverse shift, an additional tilt angle and additional aberrations. The quadrupole field adds astigmatism to the beam. To compensate the influence of each high order harmonic, at least one additional element generating the same type field is required to add to the electron optics system.
Another main problem of this multi-axis magnetic lens is the round lens field of the center sub-lens 20 is a little different from those of the peripheral sub-lenses 10 and 30. This problem results in that the charged particle beams 1, 2 and 3 respectively passing through the center sub-lens 20 and the peripheral sub-lenses 10 and 30 are not focused at the same imaging plane.
Many scientists who followed Maekawa's foot steps have tried many methods to solve these two main problems. For example, U.S. Pat. No. 6,750,455 of Lo et al. reduces the dipole field itself by using a plurality of dummy holes to improve the local structure symmetry of each sub-lens. U.S. Pat. No. 6,777,694 of Haraguchi compensates the dipole field influence by inserting a deflector group in each sub-lens hole. U.S. Pat. No. 6,703,624 of Haraguchi et al. nulls the round lens field difference among all the sub-lenses by changing the diameters of the two pole pieces or the gap size between the two pole pieces in each sub-lens to control the magnetic flux leakage. U.S. Pat. No. 6,703,624 of Haraguchi et al. and U.S. Pat. No. 7,253,417 of Frosien et al. compensate the round lens field difference by inserting an auxiliary round coil or an electrostatic lens in each sub-lens.
These previous methods were either making the magnetic conductor plate become larger, the multi-axis magnetic lens system bulky, or making it complicated. Chen et al. filed U.S. patent application Ser. No. 12/636,007 entitled “Multi-axis magnetic lens” in December 2009 to provide a better solution. Its main principle is shown in FIG. 2A by taking the sub-lens 30 in FIG. 1A as an example. In the sub-lens 30, two magnetic rings 32 and 33 are respectively inserted into the holes in the upper and lower magnetic plates 41 and 42, wherein 32 and 33 are respectively called as upper and lower pole-piece. A portion of the lower end of the upper pole-piece 32 extends inside a portion of the upper end of the lower pole-piece 33. Both of the upper pole-piece 32 and the lower pole-piece 33 are made of a magnetic material with a high permeability, and do not touch the inner sidewall of the correspondent hole. The spatial gap 34 and 35 is either a vacuum space or filled with a non-magnetic material. A space magnetic field along the optical axis 31 is generated through the space gap between the upper pole-piece 32 and the lower pole-piece 33. In FIG. 2A, the non-axisymmetric transverse field components almost become zero in the area inside the upper pole-piece 32 and the lower pole-piece 33, and are reduced by two magnetic tubes 36 and 37 to a level much lower than in FIG. 1A in the area outside the upper pole-piece 32 and the lower pole-piece 33. As an example the dipole field reduction is shown in FIG. 2B. The round lens field difference among all of the sub-lenses can be eliminated by appropriately choosing each non-magnetic gap (34 and 35) size. In this patent application, Chen et al. also provides an embodiment of multi-axis magnetic objective lens, as shown in FIG. 2C, wherein each sub-lens generates a magnetic field confined within the axial magnetic circuit gap between its upper and lower pole-pieces.
Nevertheless, it is well known that, compared with a radial-gap magnetic lens (having a radial magnetic circuit gap), this type of magnetic lens generates larger aberrations which damages spatial resolution, but requires small coil excitation (product of coil turns and coil current) which abates the issue of system overheat. In addition, a magnetic plate 50 with a plurality of through round holes is used to replace all of the individual magnetic tubes between the specimen 60 and the lower magnetic plate 42.
The present invention will adopt the main principle of Chen's multi-axis magnetic lens and further improve its performance as an immersion objective lens, so as to provide an apparatus which uses a plurality of charged particle beams to inspect a specimen in parallel with a high spatial resolution and a high throughput.